Previous work in humans and mice indicated that about 30% of mammary epithelium was able to induce angiogenesis when transplanted on the rabbit iris. Present work aims at establishing whether (a) the rat mammary gland behaves similarly (b) the rat hyperplastic nodules with high frequency of neoplastic transformation also have higher incidence of positive angiogenic response before any sign of neoplasia is detectable, (c) whether the hyperplastic lesions of the liver behave like the mammary hyperplastic epithelium and (d) whether the angiogenesis factor is present in the interstitial fluid of the tumor in vivo. BIBLIOGRAPHIC REFERENCES: Brem, S.S. and Gullino, P.M.: Angiogenesis: A marker for neoplastic transformation of mammary papillary hyperplasia. Science 195: 880-882, 1977. Gullino, P.M.: Natural history of breast cancer: Progression from hyperplasia to neoplasia as predicted by angiogenesis. Cancer 39:2697-2703, 1977.